Central sleep apnea (CSA), prevalent in approximately 40% of patients with heart failure with reduced ejection fraction (HFrEF), predicts high rates of morbidity, hospitalizations, and mortality. Adverse effects are thought to be attributable to hypoxemia, central nervous system arousal, and sympathetic nervous system activation resulting from episodic breathing disturbances and oxyhemoglobin desaturation. Until recently, the recommended treatment for CSA with HFrEF was adaptive servo-ventilation (ASV). A recent multi-national trial, however, identified an adverse effect of ASV on mortality, leaving a void in therapeutic options for patients with these co-morbid disorders. Although low flow nocturnal oxygen, which provides a physiologically sound intervention for blunting hypoxemia-associated physiological stresses and sympathetic activation, and improves short-term intermediate outcomes in patients with CSA and HFrEF, no large scale study yet has examined its long term impact on clinically important outcomes. We have assembled a team of leaders in cardiology, sleep medicine and clinical trials to conduct a pragmatic trial designed to test the hypothesis that nocturnal oxygen therapy (NOXT) in patients with CSA and HFrEF will reduce mortality and unplanned hospitalizations for worsening HF. Secondary endpoints evaluate other measures of morbidity and mortality, functional status, exercise capacity, quality of life, mood, and sleep quality. Using a double-blinded, randomized design, we will screen 2,450 patients with home sleep studies, estimating that 35% (n=858) will meet study eligibility criteria for CSA. Participants will be randomized to NOXT or sham-NOXT and undergo standardized assessments including a 6 minute walk test, complete validated questionnaires, and will be followed for study outcomes for as long as 4.5 years. Oxygen use and oxygen saturation levels will be centrally monitored using cloud-based software. Primary outcomes will be assessed using a time to first event analysis, and secondarily as recurrent event rates. Highly efficient methods for data capture and monitoring and project management will be deployed. Novel physiological markers of ventilation during sleep will be derived to identify responsive subgroups. This rigorous yet practical design will provide pivotal trial data needed to identify the role of a potentially beneficial and acceptable therapy which has yet to be tested on a large scale in a population with high morbidity and mortality, for a condition where there is no current evidence-based treatment. The sleep assessments and interventions are practically designed with the objective of facilitating their rapid uptake and use by the cardiology and sleep communities. This Data Coordinating Center application is to provide the study comprehensive, responsive, and innovative data management and study coordination services, biostatistical analysis, rigorous adjudication of study outcomes, and centralized sleep analysis and oxygen monitoring to ensure that the study meets its milestones.